Flip-chip technology is an advanced chip packaging technology to shorten the signal transmission distance between a chip and a substrate leading to better electrical performance than the conventional wire-bonding interconnection which makes flip-chip technology becomes more popular for high-speed semiconductor applications. Especially, IBM was the first to develop an innovated flip-chip technology where metal pillars are implemented to replace the conventional solder balls. Solder pastes are implemented to joint the metal pillars of a chip to the bonding pads of a substrate without changing bump shapes as the conventional solder balls during reflow, therefore, the bump pitches between metal pillars can further be reduced below 50 μm such as 30 μm to achieve higher density of bump layout. In some package products, RDL (redistribution layer) on chip can be eliminated. This technology is called MPS-C2 (Metal Post Solder-Chip Connection), the related package structure is disclosed in U.S. Pat. No. 6,229,220 B1 titling “Bump Structure, Bump Forming Method and Package Connecting Body”
As shown in FIG. 1, a conventional MPS-C2 flip-chip package 100 primarily comprises a chip 110 and a substrate 120. A plurality of metal pillars 112 are disposed on the chip 110 and extruded from the active surface of the chip 110. A plurality of bonding pads 122 corresponding to the metal pillars 112 are disposed on an upper surface 121 of the substrate 120. The metal pillars 112 are bonded to the bonding pads 122 by a plurality of solder pastes 150. Furthermore, an underfilling material 140 is formed to encapsulate the metal pillars 112, the bonding pads 122, and the solder pastes 150 where the solder pastes 150 serve as the soldering interface to electrically connect the chip 110 to the substrate 120. Since the material as well as the melting point of the solder pastes 150 are quite different from these of the metal pillars 112 and the bonding pads 122, the risk of breaking of soldering points and increasing of electrical resistance becomes higher.
The conventional MPS-C2 technology implements solder pastes 150 to mechanically and electrically connect the chip 110 to the substrate 120 where the solder pastes 150 can be chosen from low-temperature soldering materials which are different from the metal pillars 112. Since metal interdiffusion and wettability between different materials are very crucial to ensure good mechanical and electrical connections, Ni/Au are normally disposed on the surfaces of the metal pillars 112 leading to higher soldering cost. Furthermore, the solder pastes 150 are heated to the reflow temperature during the following reflow processes, the solder pastes 150 melt and become fluid where the flooding of the melting solder paste 150 easily occur when under compression or vibration caused the metal pillars 112 soldering to wrong bonding pads 122 leading to electrical failure.